The recording density of an optical recording medium is inversely proportional to the square of the diameter of the focused spot formed on the optical recording medium by an optical head incorporated within an optical information recording/reproducing apparatus that performs recording or reproduction on the optical recording medium. That is, a smaller focused spot diameter results in higher recording density. The focused spot diameter is proportional to the wavelength of the light source in the optical head and inversely proportional to the numeric aperture of the objective lens. That is, a shorter wavelength of the light source and higher numeric aperture of the objective lens result in a smaller focused spot diameter.
An optical system of an optical head suffers from various types of aberration, such as astigmatism, coma aberration, and spherical aberration, due to manufacturing error and adjustment error of optical components. For example, misalignment between the center of the incidence surface of the objective lens and the center of the exit surface thereof causes astigmatism and coma aberration, and deviation of the spacing between the incidence surface and exit surface of the objective lens from design causes spherical aberration. The occurrence of the various types of aberration in the optical system of the optical head results in a disturbed shape of the focused spot and deteriorated recording/reproducing characteristics. The astigmatism, coma aberration, and spherical aberration are inversely proportional to the wavelength of the light source, and proportional to the square, cube, and fourth-power, respectively, of the numeric aperture of the objective lens. Thus, a shorter wavelength of the light source and higher numeric aperture of the objective lens result in a narrower margin of the various types of aberrations for the recording/reproducing characteristics. Therefore, an optical information recording/reproducing apparatus provided with a light source of a shortened wavelength and an objective lens of an increased numeric aperture for improved recording density requires correction of the various types of aberration occurring in the optical system of the optical head in order to prevent deterioration in the recording/reproducing characteristics.
Known as conventional optical heads capable of correcting various types of aberration are optical heads provided with a liquid crystal optical element for correcting various types of aberration. Among them, an example of the conventional optical heads provided with a liquid crystal optical element for correcting astigmatism is described in Japanese Laid-Open Patent Application No. JP-A 2000-40249. FIG. 28 shows the configuration of the optical head disclosed in Japanese Laid-Open Patent Application No. JP-A 2000-40249. This optical head includes: a semiconductor laser 26, a polarizing beam splitter 27, a liquid crystal optical element 28, a quarter-wave plate 29, an objective lens 30, a convex lens 32, and a photo-detector 33. Light emitted from the semiconductor laser 26 serving as a light source is made incident as P-polarized light on the polarizing beam splitter 27, and is transmitted therethrough almost completely, and then outputted to the liquid crystal optical element 28. The liquid crystal optical element 28 transmits the incident light and outputs it to the quarter-wave plate 29. The quarter-wave plate 29 converts the transmitted light from linear polarized light into circular polarized light. The light transmitted through the quarter-wave plate 29 is focused on a disk 31 serving as an optical recording medium by the objective lens 30. The light reflected on the disk 31 is transmitted backward through the objective lens 30, and made incident on the quarter-wave plate 29. The quarter-wave plate 29 converts the transmitted light from the circular polarized light into linear polarized light whose polarization direction is orthogonal to that on the forward path. The light converted into the linear polarized light is transmitted backward through the liquid crystal optical element 28, and is made incident as S-polarized light on the polarizing beam splitter 27. The polarizing beam splitter 27 reflects the incident light almost completely and outputs it to the convex lens 32. The light transmitted through the convex lens 32 is received by the photo-detector 33.
The liquid crystal optical element 28 is structured to have liquid crystal polymer sandwiched between two substrates. A pattern electrode 34 is formed on a surface of one of the substrates on the liquid crystal polymer side, and an entire surface electrode is formed on a surface of the other substrate on the liquid crystal polymer side. FIG. 29 is a plan view of the pattern electrode 34 of the liquid crystal optical element 28. The pattern electrode 34 is divided into nine regions. Specifically, the pattern electrode 34 is divided into: a circular region 35a with the optical axis as center; regions 35b to 35i which are located outside of the region 35a and divided by four straight lines passing through the optical axis in units of 45 degrees in accordance with the angle around the optical axis. A dotted line in the figure indicates the effective diameter of the objective lens 30.
FIG. 30 shows relationship between the regions of the pattern electrode 34 of the liquid crystal optical element 28 and voltages respectively applied to these regions. Nine drive patterns, drive pattern A to drive pattern I, are available for the liquid crystal optical element 28. As shown in FIG. 30, selected one of drive voltages Va, Vb, and Vc is applied to each of the regions 35a to 35i, in accordance with the respective drive pattern. Here, Va>Vc>Vb, and Va−Vc=Vc−Vb=V. The transmitted light through the region(s) fed with the drive voltage Va is advanced in phase with respect to the transmitted light through the region(s) fed with the drive voltage Vc. The transmitted light through the region(s) fed with the drive voltage Vb is delayed in phase with respect to the transmitted light through the region(s) fed with the drive voltage Vc.
The drive pattern A advances the phase of the light transmitted through the regions 35c, 35d, 35g, and 35h with respect to the light transmitted through the region 35a, and delays the phase the light transmitted through the regions 35b, 35e, 35f, and 35i with respect to the light transmitted through the region 35a. On the other hand, the drive pattern E delays the phase of the light transmitted through the regions 35c, 35d, 35g, and 35h with respect to the light transmitted through the region 35a, and advances the phase of the light transmitted through the regions 35b, 35e, 35f, and 35i with respect to the light transmitted through the region 35a. Therefore, the use of the drive patterns A or E successfully provides correction of the astigmatism between the 0° direction and the 90° direction. The sign of correctable astigmatism is opposite between the drive patterns A and E.
The drive pattern C advances the phase of the light transmitted through the regions 35d, 35e, 35h, and 35i with respect to the light transmitted through the region 35a, and delays the phase of the light transmitted through the regions 35b, 35c, 35f, and 35g with respect to the light transmitted through the region 35a. On the other hand, the drive pattern G advances the phase of the light transmitted through the regions 35d, 35e, 35h, and 35i with respect to the light transmitted through the region 35a, and delays the phase of the light transmitted through the regions 35b, 35c, 35f, and 35g with respect to the light transmitted through the region 35a. Therefore, the drive patterns C and G provide correction of the astigmatism between the 45° direction and the 135° direction. The sign of correctable astigmatism is opposite between the drive patterns C and G.
The drive pattern D advances the phase of the light transmitted through the regions 35e and 35i with respect to the light transmitted through the region 35a, and delays the phase of the light transmitted through the regions 35c and 35g with respect to the light transmitted through the region 35a. On the other hand, the drive pattern H delays the phase of the light transmitted through the regions 35e and 35i with respect to the light transmitted through the region 35a, and advances the phase of the light transmitted through the regions 35c and 35g with respect to the light transmitted through the region 35a. Therefore, the drive patterns D and H provide correction of the astigmatism between 22.5° direction and 112.5° direction. The sign of correctable astigmatism is opposite between the drive patterns D and H.
The drive pattern B advances the phase of the light transmitted through the regions 35d and 35h with respect to the light transmitted through the region 35a, and delays the phase of the light transmitted through the regions 35b and 35f with respect to the light transmitted through the region 35a. On the other hand, the drive pattern F delays the phase of the light transmitted through the regions 35d and 35h with respect to the light transmitted through the region 35a, and delays the phase of the light transmitted through the regions 35b and 35f with respect to the light transmitted through the region 35a. Therefore, the drive patterns D and H provide correction of the astigmatism between 67.5° direction and 157.5° direction. The sign of correctable astigmatism is opposite between the drive patterns B and F.
The absolute amount of astigmatism correctable with the drive patterns A to H increases with the increase in the value of the voltage V. It should be noted that the drive pattern I does not provide astigmatism correction.
The correction of astigmatism with the liquid crystal optical element 28 requires selecting any of the drive patterns A to I in accordance with the direction of astigmatism to be corrected, and determining the level of voltage V in accordance with the amount of astigmatism to be corrected, so that the quality evaluation index of the reproduced signal from the optical recording medium is best improved. Japanese Laid Open Patent Application No. JP-A 2000-40249 discloses two methods as methods of determining which of the drive patterns A to I is to be used and determining the value of voltage V.
The first method involves measuring the jitter of the reproduced signal and selecting the combination of the drive pattern and the value of the voltage V so that the jitter is minimized. Eight drive patterns A to H are preliminary prepared for the drive pattern, and about 32 types of voltage values are previously prepared for the voltage V. For all the combinations, the jitter of the reproduced signal is measured, and the combination of the drive pattern and the voltage V is selected so as to minimize the jitter.
A second method involves measuring the amplitude of the reproduced signal and selecting the drive pattern so that the jitter is minimized. Eight drive patterns A to H are previously prepared for the drive pattern, about 16 types of voltage values are previously prepared for the voltage V. First, the voltage V is fixed at any one of about the 16 types of voltage values, and the amplitude of the reproduced signal, which is one of the quality evaluation indexes of the reproduced signal, is measured for all the eight drive patterns. The drive pattern is selected so that the measured amplitude of the reproduced signal is minimized. Next, the amplitude of the reproduced signal is measured for all the about 16 types of voltage V with the selected one of the eight types of drive patterns fixed. From among them, the voltage V is selected so that the amplitude of the reproduced signal is maximized.
The first method allows selecting the optimum combination of the drive pattern and voltage V which offers the best quality evaluation index of the reproduced signal. However, the first method requires long time to select the combination of the drive pattern and voltage V. On the other hand, the second method allows selecting the combination of the drive pattern and voltage V in short time. However, the second method does not necessary select the optimum combination of the drive pattern and voltage V which provides the best quality evaluation index of the reproduced signal.
In connection with the above, Japanese Laid Open Patent Application No. JP-A 2001-273663 discloses an aberration correction device. The conventional aberration correction device corrects aberration occurring on the optical path of an optical system that irradiates an optical beam to a recording medium and then guides the optical beam reflected by the recording medium. This aberration correction device includes: a liquid crystal unit provided with a first electrode layer including a plurality of divided electrodes electrically separated from each other in the same plane, a second electrode layer, and a liquid crystal element which is provided between the first and second electrode layers to cause a phase change in the light passing therethrough in accordance with the applied electric field; a detector which receives a reflected optical beam traveling through the liquid crystal unit to generate a detection signal; a voltage generator which generates voltages respectively applied to the plurality of divided electrodes; and a controller which controls aberration correction by, with the voltage applied to the predetermined divided electrode of the first electrode layer being defined as a reference voltage, changing the voltages applied to the other divided electrodes. The controller defines the reference voltage based on the change in the magnitude of the detection signal for the change of the voltages respectively applied to the plurality of divided electrodes.
Moreover, Japanese Laid Open Patent Application No. JP-A2002-14314 discloses an optical recording/reproducing device. The conventional optical recording/reproducing device includes: a voltage application electrode having a segment electrode part composed of a plurality of segment electrodes, a voltage control part formed of a conductive material and generating voltages applied to the plurality of segment electrodes by dividing an externally applied voltage by resistors of conductive material, a conduction part connecting together the segment electrode part and the voltage control part, and an insulation part preventing short-circuit between the conduction part; an opposite electrode arranged in substantially parallel to the voltage application electrode and opposed to the voltage application electrode; and a phase change layer formed of a phase changing material arranged between the voltage application electrode and the opposite electrode. The phase of light incident on the phase change layer is changed by changing the difference in the voltage between the plurality of segment electrodes and the opposite electrode.
Moreover, Japanese Laid-Open Patent Application No. JP-A2003-338070 discloses an optical head device. In this conventional example, the optical head device includes: a light source; an objective lens for focusing the light emitted from the light source onto an optical recording medium; a phase correction element provided between the light source and the objective lens to change the wavefronts of the emitted light; and control voltage generating means outputting a wavefront-changing voltage to the phase correction element. The phase correction element includes: a pair of transparent substrates with transparent electrodes formed on the surfaces thereof; and a liquid crystal layer sandwiched between the transparent substrates. Formed on the surface of at least one of the transparent substrates are: a coma aberration correction electrode which is a transparent electrode for correcting coma aberration or a spherical aberration correction electrode which is a transparent electrode for correcting spherical aberration; and an astigmatism correction electrode which is a transparent electrode for correcting astigmatism. Each of the transparent electrodes is divided into several segments.